The quality and performance of a digital fiber optic transmitter is determined by the distance over which its signal can propagate without severe distortions. The Bit Error Rate (BER) of the signal is measured at a receiver after propagation through dispersive fiber, and then the optical power required to obtain a certain BER (typically 10−12), sometimes called the sensitivity, is determined.
The difference in sensitivity between (i) at the output of the transmitter and (ii) the sensitivity after propagation through a fiber is sometimes called the dispersion penalty. This sensitivity difference is typically characterized with respect to distance over which a dispersion penalty reaches ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated source, can transmit up to a distance of ˜50 km in standard single mode fiber, at 1550 nm, before it reaches the dispersion penalty of ˜1 dB; this distance is sometimes called the dispersion limit. This particular limit is determined by the fundamental assumption that the signal is transform limited, i.e., has no time varying phase across its bits and has a bit period of 100 ps, or 1/bit rate.
Another measure of the quality of a transmitter is the absolute sensitivity after fiber propagation.
Systems for long-reach lightwave data transmission through optical fibers are disclosed in: (i) U.S. patent application Ser. No. 10/289,944, filed Nov. 6, 2002 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM; (ii) U.S. patent application Ser. No. 10/680,607, filed Oct. 06, 2003, by Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD); and (iii) U.S. patent application Ser. No. 10/308,522, filed Dec. 03, 2002 by Daniel Mahgerefteh et al. for HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY OPTICAL DISCRIMINATOR; which patent applications are hereby incorporated herein by reference. The transmitter associated with these novel systems is sometimes referred to as a Chirp Managed Laser (CML™) by Azna LLC of Wilmington, Mass.
In these new systems, an Adiabatically Frequency Modulated (AFM) source is followed by an Optical Spectrum Reshaper (OSR) which uses the frequency modulation to increase the amplitude modulated signal and partially compensates for the dispersion in the transmission fiber. An Optical Spectrum Reshaper (OSR), sometimes referred to as a frequency discriminator, can be formed by an appropriate optical element that has a wavelength-dependent transmission function. The OSR can be adapted to convert frequency modulation to amplitude modulation. Importantly, the OSR can also be adapted to convert amplitude modulation to frequency modulation.
The optical signal generated by a CML™ has an important property, i.e., that the optical carrier frequency is modulated to have the same profile in time as the information carrying amplitude modulation, albeit with a phase delay. This signal is sometimes referred to as an adiabatically chirped amplitude modulated signal in U.S. patent application Ser. No. 11/037,893 filed Jan. 18, 2005 by Yasuhiro Matsui et al. for CARRIER SUPPRESSION USING ADIABATIC FREQUENCY MODULATION (AFM), which patent application is hereby incorporated herein by reference. As is disclosed this patent application, this concomitant amplitude and frequency modulation increases the tolerance of the signal to fiber dispersion, i.e., an AFM signal can propagate a longer distance in a dispersive fiber before it is distorted and its Bit Error Rate (BER) is substantially reduced.
In one embodiment of the CML™ system, described in U.S. patent application Ser. No. 10/289,944, which patent application is hereby incorporated herein by reference, frequency modulation and amplitude modulation is achieved by modulating the bias current of an FM source, such as a Distributed Feed-Back (DFB) laser, with digital data. The resulting amplitude modulation is further enhanced by an Optical Spectrum Reshaper (OSR), sometimes also referred to as a frequency discriminator, placed after the DFB. Further improvements of the CML™ system are described in U.S. patent application Ser. No. 10/680,607 and in U.S. patent application Ser. No. 10/308,522, which two patent applications are hereby incorporated herein by reference.